The invention relates to a circuit and method in a data acquisition system for sensing minute currents (for example, currents produced by sensors) to produce very accurate digital output signals that precisely represent the currents. The invention relates more particularly to such a circuit which can be entirely integrated on a single chip, and reduces errors due to electrical system noise, input noise, amplifier offset voltages, and also reduces errors produced in an analog-to-digital converter.
Conventional techniques for integrating and digitizing minute currents (e.g. 10.sup.-12 amperes to 10 microamperes) usually include an operational amplifier and an integrating capacitor that operate to produce an output voltage which then is measured by an analog-to-digital converter. FIG. 1 shows such a system, in which a sensor 10 has the equivalent circuit shown. The purpose of the circuit of FIG. 1 is to provide a digital output signal on bus 22 that precisely represents the analog input signal i.sub.in produced by sensor 10. A conventional integrator 11 includes an operational amplifier 11A and an integrating capacitor 11B connected between the output of operational amplifier 11A and its inverting input. A reset switch 11C performs the function of resetting integrator 11 to a suitable reference voltage after each integration cycle has been completed to allow a new integration cycle to begin. Feedback causes operational amplifier 11A to change its output voltage as necessary to maintain the inverting input on conductor 12 at a virtual ground voltage. Consequently, an analog output voltage proportional to the sensor current i.sub.in is applied to the input of analog filter 13.
Analog filter 13 reduces high frequency noise at the output of operational amplifier 11A, and applies its output signal to the input of a programmable gain amplifier 14. The purpose of programmable gain amplifier 14, which must be an expensive, high accuracy, component, is to maximize use of the dynamic range of analog-to-digital converter 18. The output of programmable gain amplifier 14 is applied to the input of a sample and hold amplifier 16, which holds an amplified analog voltage on conductor 16A to accurately represent sensor input current i.sub.in. Resetting of integrating capacitor 11B is synchronized with sampling of the output of programmable gain amplifier 14 by means of sample and hold amplifier 16.
Block 18 contains an analog-to-digital converter (ADC) that converts the voltage 16A to the desired digital output voltage 22. The voltage 16A is applied to the non-inverting input of comparator 19. ADC 18 includes a digital-to-analog converter (DAC) 21, the output of which is connected to the inverting input of comparator 19. Bus 22A is connected to the digital inputs of DAC 21. A logic circuit 20 performs the function of producing successive digital outputs representing the voltage 16A on bus 22A until the inverting input of comparator 19 is equal to the voltage on conductor 16A. Bus 22 provides the final digitized output of the data acquisition system.
Shortcomings of the prior art circuit of FIG. 1 include the fact that analog filter 13, programmable gain amplifier 14, and sample and hold circuit 16 all are expensive circuits. Furthermore, electronic noise of operational amplifier 11A and ADC 18 and offset errors of operational amplifier 11A and ADC 18 tend to reduce the accuracy of the digital output on bus 22.
Other shortcomings of the prior art circuit of FIG. 1 include reset kT/C errors and reset charge injection errors. Various errors associated with analog-to-digital converter 18 also significantly reduce the accuracy of the digital output on bus 22. Voltage coefficients associated with integrating capacitor 11B of the circuit of FIG. 1 produce nonlinearities that reduce the accuracy of the digital output.
It can be seen that there is an unmet need for a low cost circuit that rapidly and accurately digitizes minute analog input signals generated by various transducers.